A technique has been developed that determines simultaneously solubilities and diffusivities of gases in molten or thermally softened polymers. Henry's low was found to hold up to 20 atm. In addition it was found that pressure had no appreciable effect on diffusion coefficients up to 20 atm. Solubilities and diffusivities were determined for systems involving nitrogen, helium, carbon dioxide, and argon in polyethylene, polyisobutylene, and polypropylene. In addi. tion, solubilities were also determined for the preceding gases in polystyrene and polymethylmethacrylate. Other data were also obtained for neon, krypton, and monochlorodiflouromethane in various polymers.and diffusivity data for gases in softened or chemical engineer. Such data are of particular interest in polymer finishing processes, where the molten polymer is blanketed with inert gas; in certain specialized operations such as foam extrusion and fluidized-bed coating; and finally in the production of condensation polymers, where the small molecule split out in the condensation reaction must be continuously removed from the polymer melt in order to achieve a high degree of polymerization.The only studies found in the literature that considered solubilities and diffusivities of gas in softened or molten polymers were those of Lundberg, Wilk, and Huyett (1 to 3 ) and Newitt and Weale ( 4 ) . The fonner investigators considered the systems nitrogen-polyethylene, methanepolyethylene, and methane-polystyrene. Their polymer samples were constrained in a sintered steel cylinder placed within a pressure vessel. The principal deficiency in their work was that they assumed their data to hold for the average of their initial and final pressures even though pressure ranges were sizable.Newitt and Weale ( 4 ) studied the solution and diffusion of hydrogen, nitrogen, carbon dioxide, and ethylene in polystyrene. These workers used sosption experiments and diffusivity measurements by movement of a color boundary to determine their data. A defect in their procedure was their failure to preheat the gases used in the experimental work.The present work was undertaken to determine simultaneously solubility and diffusivity data for a number of gas-polymer systems.The apparatus (Figure 1) used permitted the simultaneous determination of solubility and diffusion coefficient in a single sorption experiment. In principle, the equipment and procedure were similar to those of Lundberg, Wilk, and Huyett and of Newitt and Weale.The diffusion cells (Figure 2 ) were constructed from 2?4-in. cold-rolled bar stock. The premolded polymer samples were contained in a cylindrical 1 in. by 1 in. diffusion cell cavity so that diffusion of gas occurred downward through the surface of the sample. The solid samples were molten sO1ubiliY poymers are of considerable importance to the Preston L. Durrill is at Radford College, Radford, Virginia.generally about 3/4 in. in height and slightly less than 1 in. in diameter.The diffusion cells were heated in a fluidized bed which was both fluidized and...
Solubilities and diffusivities of various gases (helium, nitrogen, carbon dioxide, argon, neon, krypton, and monochlorodifluoromethane) in molten or thermally softened polymers (polyethylene, polypropylene, polyisobutylene, polystyrene, and polymethylmethacrylate) have been correlated with structural characteristics, temperature, and pressure. Temperature dependence of both Henry's Law constants and diffusivities were of the Arrhenius equation form. No appreciable effect of pressure was found for either Henry's Law constants or diffusivities up to 300 atm. Earlier correlations for Henry's Law constants in solid polymer systems were found to be inapplicable for molten and thermally softened polymers. New correlations were developed individually for the latter systems. The correlating factor used was the gos LennardJones force constant. Existing correlations for diffusivities were also found not to apply to molten and thermally softened systems. New correlations were again developed on an individual polymer basis. These related diffusivity to gas Lennard-Jones collision diameter or molecular diameter. Generalized correlations were also developed that held for a number of polymers.These were for both Henry's Law constants and diffusivities.In an earlier paper (1) solubility and diffusivity data were presented for gases absorbed into molten or thermally softened polymer systems. The intent of this paper is to use these data together with other literature information ( 2 to 10) to correlate solubilities and difFusivities with temperature, pressure, and gas and polymer structural characteristics for molten or thermally softened polymer systems.
The mechanism of solid state polycondensation has been subjected to a fundamental analysis. Equations were formulated for combined diffusion and chemical reaction for two separate situations. One was for solid state polycondensation in polymer flakes or chips. The other dealt with polymer powders. The resultant solutions related molecular weight changes to rote functions. A technique for deriving the rate functions from experimental data is described.Solid state polycondensations were then studied for nylon 66, nylon 6-10, and polyethylene terephthalate. These data which ranged from 120 to 200°C. were tested with various mechanisms. The most appropriate one was found to be that developed in the present work. Chemical reaction was found to be the rate controlling step in solid state polycondensation in nylon 66, polyethylene terephthalate, powders of nylon 6-10 and larger particles of nylon 6-10 at and above 160°C. Diffusion of byproduct through the solid was the rate controlling step for larger particles of nylon 6-10 a t temperatures below 160°C. Thermograms of nylon 6-10 indicated morphological changes which possibly influenced the behavior of the larger nylon 6-10 particles. The Arrhenius relation was fitted to the situations where chemical reaction controlled.The thermally induced solid state polymerization of condensation polymers is a phenomenon that is well known in industry. The process is carried out by heating (but not melting) the condensation polymers in an inert gas atmosphere. In condensation polymers an equilibrium exists (192) Pi + P 2 e P 3 + B where Pi, Pz are polymer chains which combine to form Pa which is a longer ohain, and B a byproduct which is small molecule such as water. In essence then, solid state polymerization actually involves taking off the byproduct and driving the reaction to the right. There then can be three possible rate determining steps:1. Chemical reaction 2. Diffusion of byproduct molecules in the solid polymer 3. Diffusion of byproduct molecules from the solid polymer surface to the inert gas.Most of the previous investigations ( 2 to 9 ) of solid state polymerization have generally neglected the mechanism aspect. One exception was the work of Lee and Griskey (9) who found that the rate controlling steps took place within the polymer itself. This was shown in two ways. First, varying the flow rate of nitrogen through the reactor had little effect on polymerization. This indicated that diffusion into the gas was not a rate controlling step. Furthermore, it was found that the rate of polymerization could be described by the equation A recent study has treated this combined chemical reaction and diffusion in a more fundamental manner. The study first considered two possible geometric situations, polymer flakes and polymer powders. The latter material will more closely resemble spheres while the former is closer to the geometry of plane sheets (see Figures 1 and 2 ) .In
SynopsisThe thermally induced solid-state polymerization of 66 nylon was investigated. It was found that the rate-controlling step in the process is chemical reaction. A mechanism of the form, rate = kt" was shown to hold. The reaction rate constant was found to be k = 1.53 X 1O'O exp [-12,96O/RT]. Activation energy determined in this work compared closely to that determined for nylon 6 solidstate polymerization.
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